As far as paying for advertising goes on Facebook I don't rate it I tried it for a company who targets 18-30 yr olds the prime age for users on facebook and most of them just liked the thing we did get traffic but no sales.

I do thing all social networking helps your website as it helps increase the word of your website and will generate new clicks etc

I only really worry with Facebook, Ecademy, Twitter, Digg, Stumble, 4 Networking, UKBF and this forum.